Field emission electron source

ABSTRACT

There is provided a field emission electron source at a low cost in which electrons can be emitted with a high stability and a high efficiency and a method of producing the same. In the field emission electron source, a strong electric field drift part  106  is formed on the n-type silicon substrate on the principal surface thereof and a surface electrode  107  made of a gold thin film is formed on the strong electric field drift part  106 . And the ohmic electrode  2  is formed on the back surface of the n-type silicon substrate  101 . In this field emission electron source  110 , when the surface electrode  107  is disposed in the vacuum and a DC voltage is applied to the surface electrode  107  which is of a positive polarity with respect to the n-type silicon substrate  101  (ohmic electrode  2 ), electrons injected from the n-type silicon substrate  101  are drifted in the strong electric field drift part  106  and emitted through the surface electrode  107 . The strong electric field drift part  106  comprises a drift region  161  which has a cross section in the structure of mesh at right angles to the direction of thickness of the n-type silicon substrate  1 , which is an electrically conductive substrate, and a heat radiation region  162  which is filled in the voids of the mesh and has a heat conduction higher than that of the drift region  161.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.09/404,656, filed Sep. 24, 1999, now U.S. Pat. No. 6,590,321, thedisclosure of which is expressly incorporated by reference herein in itsentirety.

DETAILED DESCRIPTION OF THE INVENTION

1. Field of the Invention

This invention relates to a field emission electron source for emittingelectrons due to the field emission using a semiconductor materialwithout heating and a method of manufacturing the same. Moreparticularly, the present invention relates to a field emission electronsource applicable to a planar light emitting apparatus, a displayapparatus, and a solid vacuum device, and a method of producing thesame.

2. Prior art

As field emission electron sources, those using the so-called Spindttype electrode such as disclosed in, for example, U.S. Pat. No.3,665,241 are well known. The Spindt type electrode comprises asubstrate having a multitude minute emitter chips of a triangularpyramid shape disposed thereon and gate layers that have emission holesthrough which tips of the emitter chips are exposed and are insulatedfrom the emitter chips. In this structure, a high voltage is applied ina vacuum atmosphere to the emitter chips as negative electrode withrespect to the gate layer, electron beams can be emitted from the tipsof the emitter chips through the emission holes.

However, the production process of the Spindt type electrode iscomplicated and it is difficult to make a multitude of emitter chips ofa triangular pyramid shape with high accuracy and hence, difficult tomake a device of large emission area when applying this technology to,for example, a planar light emitting apparatus or a display apparatus.Also with the Spindt type electrode, since the electric field isconcentrated on the tip of the emitter chip, emitted electrons ionizevarious residual gases into positive ions where the degree of vacuum islow and the residual gas exists in the vicinity of the tips of theemitter chips. Therefore, the positive ions impinge on the tip of theemitter chips and eventually damage the tips of the emitter chips,resulting in such problems that the current density and efficiency ofthe emitted electrons become unstable and the service life of theemitter chips decreases. Thus, the Spindt type electrode has such adrawback that the atmosphere in which it is used must be pumped to ahigh degree of vacuum (10⁻⁵ Pa to 10⁻⁶ Pa) in order to avoid theproblems described above, resulting in higher cost and difficulthandling.

In order to overcome the drawback described above, field emissionelectron sources of MIM (Metal Insulator Metal) type and MOS (MetalOxide Semiconductor) type have been proposed. The former is a fieldemission electron source of a planar configuration having a laminatedstructure of metal-insulation film-metal and the latter is the samestructure one of a metal-oxide film-semiconductor. However, it isnecessary to reduce the thickness of the insulation film or the oxidefilm in order to improve the electron emitting efficiency to therebyincrease the number of electrons emitted with these types of fieldemission electron sources, while making the insulation film or the oxidefilm too thin may lead to dielectric breakdown when a voltage is appliedbetween the upper and lower electrodes of the laminated structuredescribed above. Thus there has been such a problem that, in order toavoid the dielectric breakdown of the insulator film, the electronemitting efficiency (pullout efficiency) cannot be made too high becausethere is a limitation on the reduction of the thickness of theinsulation film or the oxide film.

A different field emission electron source has recently been proposed inJapanese Patent Kokai Publication No. 8-250766. According to thispublication, the field emission electron source is made by using asingle-crystal semiconductor substrate such as a silicon substrate,forming a porous semiconductor layer (a porous silicon layer, forexample) by anodization of one surface of the semiconductor substrate,and forming a surface electrode made of a thin metal film on the poroussemiconductor layer. A voltage is adapted between the semiconductorsubstrate and the surface electrode to cause the field emission electronsource (semiconductor cold electron emitting device) to emit electrons.

However, in the structure disclosed in Japanese Patent Kokai PublicationNo. 8-250766, there is such a drawback that the popping phenomenon islikely to occur during electron emission. In the field emission electronsource in which the popping phenomenon is likely to occur duringelectron emission, the unevenness in amount of electrons emitted islikely to occur. Thus, when this type of field emission electron sourceis used in a planar light emitting device and a display apparatus, thereis such a drawback that the light is not emitted uniformly.

Then, the inventors studied whole-heartedly the above drawbacks andfound out that in the field emission electron source as disclosed inJapanese Patent Kokai Publication No. 8-250766, since a porous siliconlayer formed by making the entire surface of the single crystalsubstrate on the principal surface side porous constructs a strongelectric field drift layer into which electrons are injected, the strongelectric field drift layer has a heat conductivity lower than that ofthe crystal substrate and the field emission electron source has a highthermal insulating characteristics, which results in that thetemperature of the substrate rises relatively largely when voltage isapplied and current is flown. Further the inventors found out thatelectrons are thermally excited and electrical resistivity of thesingle-crystal semiconductor substrate decrease when the temperature ofthe substrate increases, accompanied by increase of the amount ofelectrons emitted. Therefore, this structure is susceptible to thepopping phenomenon during electron emission leading to unevenness inamount of electrons emitted.

Based on the above findings, the present invention has beenaccomplished. That is, the object of the present invention is to providea field emission electron source capable of achieving a stable emissionof electrons with high efficiency at a low cost and a method ofproducing the same.

THE SUMMARY OF THE INVENTION

In order to achieve the above-mentioned object, according to a firstaspect of the present invention, there is provided a field electronsource comprising an electrically conductive substrate having principalsurfaces; a strong electric field drift layer formed on one of theprincipal surfaces of said electrically conductive substrate and asurface electrode made of a thin electrically conductive film which isformed on said strong electric field drift layer, wherein a voltage isapplied to said surface electrode used as a positive electrode withrespect to said electrically conductive substrate, thereby electronsinjected from said electrically conductive substrate being drifted insaid strong electric effect drift layer and emitted through said surfaceelectrode, characterized in that said strong electric field drift layercomprises at least a) semiconductor crystal regions formed in a mannerto stand up vertically on said principal surface of the electricallyconductive substrate and b) semiconductor micro-crystal regions havingnano-structures intervened between the semiconductor crystal regionscoated with an insulating film which has a thickness smaller than thecrystal grain size of said semiconductor micro-crystal region and isformed on the surface of the semiconductor micro-crystal. Therefore, 1)the dependency on the degree of vacuum of the electron emissioncharacteristic is low and no popping phenomenon occurs during theelectron emission. Also the electrons can be emitted with a highstability and a high efficiency. 2) As the electrically conductivesubstrate, the semiconductor substrate such as a single-crystal siliconsubstrate and the substrate such as a glass substrate with a conductivefilm formed thereon can be used, in which case it is made possible toachieve larger emission area and lower production cost than in the caseof using the conventional porous semiconductor layer and of theSpindt-type electrode, as in the conventional example.

In the present invention, said semiconductor crystal is preferablypolysilicon. But other single crystal, poly-crystal and amorphoussemiconductor, for example, poly-crystal semiconductor of IV group,IV—IV group compound semiconductor such as SiC, III-V group compoundsemiconductor such as GaAs, GaN and InP, and II-VI group semiconductorsuch as ZnSe may be used.

In the present invention, the semiconductor micro-crystal region isformed by making the single crystal or poly-crystal semiconductor porousby the anodization, which constructs a drift region; the details thereofare described in U.S. patent application Ser. No. 09/140,647, now U.S.Pat. No. 6,249,080, the content whereof is incorporated in thisspecification by reference. The insulating film preferably made of anoxide film or a nitride film.

In order to achieve the above-mentioned object, according to a secondaspect of the present invention, there is provided a field electronsource comprising an electrically conductive substrate, a strongelectric field drift part formed on one of the principal surface of saidelectrically conductive substrate and a surface electrode of a thinmetal film formed on said strong electric field drift part, wherein a DCvoltage is applied to said surface electrode used as a positiveelectrode with respect to the electrically conductive substrate, therebyelectrons injected from the electrically conductive substrate beingdrifted in said strong electric effect drift part and emitted throughsaid surface electrode, wherein the strong electric field drift partpreferably comprises drift regions in which the electrons are driftedand heat radiation regions which have a heat conductivity higher thanthat of the drift region, the drift regions and the heat radiationregions are mixed and distributed uniformly. In a typical case, thedrift regions has a mesh-like cross section at right angles to thedirection of thickness of the electrically conductive substrate and, theheat radiation regions which are built up in the mesh openings.Therefore, the heat generated in the drift region is radiated throughthe heat radiation region in the strong electric field drift part andthus, no popping phenomenon occurs during the electron emission and theelectrons can be emitted with a high stability and a high efficiency.

The drift region may be a layer made by alternately laminating layerswhose porosity are different from each other in the direction ofthickness of the electrically conductive substrate, thereby theefficiency of the electron emission can be enhanced. And said driftregion may be a layer whose porosity changes continuously in a directionof thickness of the electrically conductive substrate, thereby theefficiency of the electron emission can be enhanced.

The openings or void of the mesh-like drift region is preferably in theshape of a minute polygon or a minute circle.

The drift regions and heat radiation regions may be selected from thegroup consisting of a single crystal, poly-crystal and amorphous ofsilicon or silicon carbide. The heat radiation region is preferably asilicon or silicon carbide with an insulating film on the surfacethereof and therefore, the heat radiation region has a high heatconduction characteristic and a electrical insulating characteristic,resulting in the increase of heat radiation. The insulating film ispreferably an oxide film or a nitride film.

The surface electrode is preferably made of a thin metal, buttransparent and conductive films of ITO, SnO2 and ZnO2 can be used forthe surface electrode.

The electrically conductive substrate is preferably a substrate on oneof the principal surface of which the electrically conductive film isformed and therefore, it is made possible to achieve larger emissionarea and lower production cost than in the case of using a semiconductorsubstrate such as a single-crystal silicon substrate as an electricallyconductive substrate.

In order to produce the field emission electron source, a part of thesemiconductor region on the principal surface of the electricallyconductive substrate is made porous by anodization in the direction ofthickness, and then the semiconductor region and the poroussemiconductor region are oxidized to form a heat radiation region and adrift region, finally a surface electrode made of a thin metal filmbeing formed on the strong electric field drift part comprising thedrift region and the heat radiation region.

Because a part of the semiconductor region on the principal surface ofthe electrically conductive substrate is made porous and then oxidized,the drift region and the heat radiation region can be formed using thesame semiconductor material. Therefore, it is not necessary to form thedrift region and the radiation region separately from the beginning ofthe preparation and the shape of the pattern of the drift region and theheat radiation region can be easily controlled. As a result, a fieldemission electron source in which no popping phenomenon occurs duringthe electron emission and electrons can be emitted with a high stabilityand a high efficiency can be achieved at a low cost.

Where said anodization is effected, 1) if the anodization is carried outafter a column-like poly-crystal semiconductor layer stood up verticallyon the surface of the electrically conductive substrate was made,mixture structure of the semiconductor crystal regions and thesemiconductor micro-crystal regions can be easily made. 2) If the maskhas cross-section shape of a mesh with openings like a minute polygon isarranged on an area on which the heat radiation region is to be formedon the semiconductor region and then, the anodization is effected, withthe result that only the part of the semiconductor region on the surfaceof the electrically conductive substrate which corresponds to the driftregion can be made porous by anodization. Also, in the case of that themask has a cross section of a mesh with openings in the shape of aminute circle is arranged on the area on which the heat radiation regionis to be formed on the semiconductor region and then, the anodization iseffected, only the part of the semiconductor region on the surface ofthe electrically conductive substrate which corresponds to the driftregion can be made porous by anodization.

Where said anodization is effected, 3) the magnetic field is applied tothe electrically conductive substrate during the anodization in such amanner that the rate making the semiconductor region porous in thevertical direction to the one surface of the electrically conductivesubstrate is much faster than that in the other directions, with theresult that the anisotropy in the rate of making the semiconductorregion porous is enhanced. That is, in the region which is to be a driftregion by oxidation after making porous, the anisotropy in the formingrate of the porous layer during the anodization is enhanced. Therefore,the controllability in the shape in the horizontal direction and in thedirection of thickness of the drift region can be enhanced, with theresult that the minute patterns of the-drift region and the heatradiation region can be formed with a good controllability in thedirection of thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof and the accompanying drawingsthroughout which like parts are designated by like reference numerals,and in which:

FIG. 1 is a schematic diagram for explaining the principle of mechanismof electron emission in the filed emission electron source according tothe present invention;

FIG. 2 is a cross sectional view showing the first embodiment of thepresent invention;

FIGS. 3A, 3B, 3C and 3D are cross sectional views showing major stepsfor explaining the production process of the field emission electronsource of the first embodiment;

FIG. 4 is a schematic diagram for explaining the measuring principle ofemitted electrons of the field emission electron source of the firstembodiment;

FIG. 5 is a graph showing a voltage-current characteristic of the fieldemission electron source of the first embodiment;

FIG. 6 is a graph of Fowlev-Nordheim plot of the data of FIG. 5;

FIG. 7 is a graph showing a change in current with time in the firstembodiment;

FIG. 8 is a graph showing dependency of the current density on thedegree of vacuum in the first embodiment;

FIG. 9 is a schematic diagram for explaining the energy distribution ofthe emitted electrons in the first embodiment;

FIGS. 10A and 10B are a schematic vertical cross sectional view and aschematic transverse cross sectional view of the second embodiment,respectively;

FIGS. 11A, 11B and 11C are cross sectional views showing major steps forexplaining the production process of the second embodiment;

FIG. 12 is a plan view of a photo-mask for explaining the productionprocess of the second embodiment;

FIG. 13 is a schematic vertical cross sectional view showing the thirdembodiment;

FIG. 14 is a schematic vertical cross sectional view showing the fourthembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 2 is a schematic diagram showing the configuration of a fieldemission electron source 10 according to this embodiment, FIGS. 3A to 3Dare cross sectional views of major steps of producing the field emissionelectron source 10. In this embodiment, an n-type silicon substrate 1((100)-substrate having a resistivity of about 0.1 Ωcm) is used for anelectrically conductive substrate.

As shown in FIG. 2, the field emission electron source according to thisembodiment includes a polysilicon layer 5 oxidized by the rapid thermaloxidation technique on the principal surface of the n-type siliconsubstrate, aporous polysilicon layer 6 oxidized by the rapid thermaloxidation technique on the polysilicon layer 5 and a gold thin film,which is a thin metal film, formed on the porous polysilicon layer 6.And an ohmic electrode 2 is formed on the back surface of the n-typesilicon substrate 1.

In this embodiment, a n-type silicon substrate 1 is used as anelectrically conductive substrate. The electrically conductive substrateforms a negative electrode of the field emission electron source 10 andsupports said porous polysilicon layer 6 in vacuum. Further, whenvoltage is applied to the electrically conductive substrate, theelectrons are injected to the porous polysilicon layer.

And said porous polysilicon layer 6 acts as a strong electric fielddrift layer where the electrons injected from the electricallyconductive substrate are drifted when voltage is applied between theelectrically conductive substrate and the thin metal film.

A method of making the field emission electron source will be describedbelow with reference to FIG. 3.

First the ohmic electrode 2 is formed on a back surface of the n-typesilicon substrate 1, and then an undoped polysilicon layer 3 of about1.5 μm in thickness is formed on a front surface of the n-type siliconsubstrate 1 opposite to the back surface, thereby to obtain a structureas shown in FIG. 3A. The polysilicon layer 3 is formed by the use ofLPCVD process, using a vacuum of 20 Pa, a substrate temperature of 640 °C., and a floating silane gas at 600 sccm.

After the undoped polysilicon layer 3 is formed, the polysilicon layer 3is subjected to anodization with a constant current while beingirradiated with light. During this anodization, a liquid electrolytemade by mixing a 55 wt % aqueous solution of hydrogen fluoride andethanol in a proportion of about 1:1 is used and a platinum electrode(not shown) is used as a negative electrode and the n-type siliconsubstrate 1 (ohmic electrode 2) is used as a positive electrode. By thisanodization, a porous polysilicon layer 4 (hereinafter referred to asPPS layer 4) can be obtained as shown in FIG. 3B. In this embodiment,the anodization process was conducted under conditions of a constantcurrent density of 10 mA/cm² and duration of anodization being 30seconds, while irradiating the surface of the polysilicon layer 3 withlight by means of a 500W tungsten lamp during the process ofanodization. As a result, the porous polysilicon layer 4 of about 1 μmin thickness was formed in this embodiment. While a part of thepolysilicon layer 3 is made porous in this embodiment, the entirepolysilicon layer 3 may be made porous.

Then, by effecting the rapid thermal oxidation (RTO) to the PPS layer 4and the polysilicon layer 3, a structure shown in FIG. 3C is obtained.Reference numeral 5 in FIG. 3C denotes a part of the polysilicon layerprocessed by the rapid thermal oxidation and reference numeral 6 denotesa part of the PPS layer processed by the rapid thermal oxidation(hereinafter referred to as RTO-PPS layer 6). The rapid thermaloxidation process was conducted at an oxidation temperature of 900 ° C.for the oxidation period of one hour. In this embodiment, since the PPSlayer 4 and polysilicon layer 3 are oxidized by the rapid thermaloxidation, the layers can be heated up to the oxidation temperature inseveral seconds, thus making it possible to suppress entrainmentoxidation taking place when charging into a furnace in case theconventional oxidation apparatus of furnace tube type is used.

Then the thin gold film 7, which is a metal thin film, is formed on theRTO-PPS layer 6 by, for example, evaporation, thereby to obtain thefield emission electron source 10 having a structure shown in FIGS. 3Dand 2. While the thickness of the thin gold film 7 is about 10 nm inthis embodiment, the thickness is not limited to a particular value. Thefield emission electron source 10 forms a diode with the thin gold film7 serving as a positive electrode (anode) and the ohmic electrode 2serving as a negative electrode (cathode). While the thin metal film isformed by evaporation in this embodiment, the method of forming the thinmetal film is not limited to evaporation and the thin metal film may beformed by sputtering.

Now characteristics of the field emission electron source 10 of thisembodiment will be described below.

The field emission electron source 10 is housed in a vacuum chamber (notshown) and a collector electrode 21 (collector for emitted electrons) isdisposed at a position so as to confront the thin gold film 7 as shownin FIG. 4. Inside of the vacuum chamber is evacuated to a degree ofabout 5×10⁻⁵ Pa. A DC voltage Vps is applied with the thin gold film 7at a positive polarity with respect to the ohmic electrode 2 (i.e.n-type silicon substrate 1), and a DC voltage Vc is applied with thecollector electrode 21 at a positive polarity with respect to the thingold film 7. Measurements of the diode current Ips flowing between thethin gold film 7 and the ohmic electrode 2, and the electron emissioncurrent Ie flowing between the collector electrode 21 and the thin goldfilm 7 due to the emission of electrons e⁻from the field emissionelectron source 10 through the thin gold film 7 (alternate dash and dotline in FIG. 4 represents the emitted electron current) are shown inFIG. 5.

In FIG. 5, the DC voltage Vps is plotted along the horizontal axis andthe current density is plotted along the vertical axis. Curve A in thedrawing represents the diode current Ips and curve B represents theelectron emission current Ie. The DC voltage Vc is set constant at 100V.

As will be seen from FIG. 5, the electron emission current Ie wasobserved only when the DC voltage Vps was positive, while both the diodecurrent Ips and the electron emission current Ie were increased as theDC voltage Vps was increased. Specifically, when the DC voltage Vps was15 V, current density of the diode current Ips was about 100 mA/cm² andthe current density of the electron emission current Ie was about 10μA/cm². This value of the electron emission current Ie was greater thanthat obtained with a field emission electron source using the poroussilicon layer formed by making the surface of a single crystal siliconsubstrate porous as a strong electric field drift layer, describedpreviously in conjunction with the prior art. According to the“Electronic Information & Telecommunications Association ED96-141,pp41-46”, they described about 40 mA/cm² for the current density of thediode current Ips and 1 μA/cm² for the current density of the electronemission current Ie when the DC voltage Vps was 15 V. Thus, thisembodiment of the present invention is effective to provide the fieldemission electron source exhibiting a high efficiency of electronemission.

FIG. 6 shows Fowlev-Nordheim plot of the electron emission current Ieversus DC voltage Vps. The fact that the plots lie on a straight lineindicates that the electron emission current Ie caused by the emissionof electrons is due to the well-known quantum tunneling effect.

FIG. 7 is a graph showing the diode current Ips and the electronemission current Ie of the field emission electron source of thisembodiment with change in time. Time is plotted along the horizontalaxis and the current density is plotted along the vertical axis, whilecurve A shows the diode current Ips and curve shows the electronemission current Ie. Shown in FIG. 7 is the result obtained by settingthe DC voltage Vps constant at 15 V and the DC voltage Vc constant at100 V. As will be seen from FIG. 7, any popping phenomenon is notobserved in both the diode current Ips and the electron emission currentIe with the field emission electron source 10 of this embodiment, sothat the diode current Ips and the electron emission current Ie can bemaintained substantially constant with time.

Such a characteristic of stable electron emission current Ie with lesschronic change achieved by employing the configuration of the presentinvention cannot be achieved with the conventional field emissionelectron source based on the MIM system or single-crystal siliconsubstrate of which surface is made porous.

Now the dependency on the degree of vacuum of the electron emissioncurrent Ie of the field emission electron source 10 of this embodimentwill be described below. FIG. 8 shows the diode current Ips and theelectron emission current Ie changing as a function of the degree ofvacuum of the argon atmosphere which surrounds the field emissionelectron source 10 of this embodiment. In FIG. 8, the degree of vacuumis plotted along the horizontal axis and the current density is plottedalong the vertical axis. Curve A in the drawing represents the diodecurrent Ips and curve B represents the electron emission current Ie.FIG. 8 shows that a substantially constant electron emission current Iecan be obtained in a range of degrees of vacuum from 10⁻⁴ Pa to about 1Pa, indicating an insignificant dependence of the electron emissioncurrent Ie on the degree of vacuum. Thus, because of the low dependencyon the degree of vacuum of the electron emission current Ie of the fieldemisison electron source 10 of this embodiment, stable emission ofelectrons of high efficiency can be maintained even when the degree ofvacuum changes to some extent. Thus, because the satisfactory electronemission characteristic can be achieved even with a low degree ofvacuum, it is not necessary to use the field emission electron sourceunder a high degree of vacuum, and it is made possible to produce anapparatus which uses the field emission electron source 10 at a lowercost with handling thereof made easier.

Next, the mechanism of the electron emission in the field emissionelectron source according to this embodiment of the present inventionwill be described below.

First, in order to study the mechanism of the electron emission, whenthe cross section of the PPS layer 4 of the specimen shown in FIG. 3Bafter anodization was observed with a transmission type electronmicroscope (TEM), it was confirmed that the micro-crystal silicon layerhaving nano-structures (about 5 nm in the diameter) was grown around thecolumnar polysilicon. And when the cross section of the specimen shownin FIG. 3A after forming the polysilicon layer 3 was observed with aTEM, it was confirmed that the polysilicon layer 3 was composed ofaggregates (columnar structure) of the fine columnar grains (crystalgrain) oriented in the direction of the film growth (in the verticaldirection in FIG. 3A). With comparison of these observation results withTEM, it is assumed that anodization of the polysilicon layer 3progresses faster at the boundary of the grain, that is, anodizationprogresses in the direction of thickness between the columns of thecolumnar structure and the columnar silicon grain structure remainsafter anodization. This is because the rate of the formation of theporous layer (PPS layer 4) is faster than that in the case that theporous silicon layer is formed by anodizing the single-crystal siliconsubstrate and the space density of the micro-crystal silicon layerhaving nano-structures where the quantum confinement effect is developedis reduced, while the relatively large columnar grains remain. In thiscase, judging from the control of electric conductivity and thestructural and heat stability, because the columnar grain structureremains, the porous poly-crystal silicon formed by anodizing thepolysilicon layer in the columnar structure seems to have betterproperties than those of the porous poly-crystal silicon formed byanodizing the bulk polysilicon layer.

From the above-mentioned results of TEM observation, the porouspolysilicon layer 6 (RTO-PPS layer 6) oxidized by a rapid thermaloxidation as shown in FIG. 3D, that is, the strong electric field driftlayer is supposed to comprise at least, a polysilicon 61 which iscolumnar semiconductor crystal, a thin silicon oxide film 62 formed onthe polysilicon 61, a micro-crystal silicon layer 63 which is asemiconductor micro-crystal intervened between the columnar polysilicon61, and a silicon oxide film 64 which is formed on the surface of themicro-crystal silicon layer 63 and is an insulating film having athickness smaller than the crystal grain size of said micro-crystalsilicon layer 63, as shown in FIG. 1.

Therefore, in the field emission electron source 10 according to thisembodiment, the electrons seem to be emitted in the following mechanism.When the DC voltage Vps, applied to the thin gold film 7 which is of apositive polarity with respect to the n-type silicon substrate 1,reaches a predetermined threshold value, electrons e⁻ are injected fromthe n-type silicon substrate 1 into the RTO-PPS layer 6 by thermalexcitation. At this time, since most of electric field applied to theRTO-PPS layer 6 is applied across the silicon oxide layer 64, theinjected electrons e⁻ are accelerated by the strong electric fieldapplied across the silicon oxide layer 64 and are drifted through thespace between the polysilicon 61 in the RTO-PPS layer 6 toward thesurface in the direction of the arrow A in FIG. 1 (upward in FIG. 1). Inthis case, the drift length of the electrons in the RTO-PPS layer isvery long as compared with the grain size of the micro-crystal siliconlayer 63 as described below, the electrons reach the surface of theRTO-PPS layer 6 with almost no collision. The electrons e⁻ which havereached the surface of the RTO-PPS layer 6 are hot electrons having akinetic energy much higher by several kT or much more than that in thestate of thermal equilibrium and easily penetrate the thin gold film 7through the oxide layer at the top surface of the RTO-PPS layer 6 due totunneling, thereby to be emitted to into the vacuum.

In the field emission electron source 10 of this embodiment, asdescribed above with reference to FIG. 7, the electrons can be emittedwithout the occurrence of the popping noise and with a high efficiencyand a high stability. This is because it is supposed that the surface ofeach grain in the RTO-PPS layer is made porous but the core of eachgrain (polysilicon 61 in FIG. 1) retains a crystal state and it is alsosupposed that heat generated by applying voltage transmits along thecrystal (polysilicon 61 in FIG. 1) and radiates to the outside,therefore temperature rise of RTO-PPS layer being suppressed.

Based on the discussion above, it is supposed that the RTO-PPS layer 6,which is a strong electric field drift layer, has a semi-insulatingcharacteristic to make a strong electric field lie. And it is supposedthat in the RTO-PPS layer 6, the electron scattering is small and thedrift length is long, the heat conductivity being high enough tosuppress the thermorunaway of the diode current Ips. Therefore, it ismade possible to achieve stable electron emission with high efficiency.

The facts that support the mechanism of the electron emission due to thetunneling effect of the hot electrons as described above will bedescribed. Such facts are 1. Strong electric field effect at thesurface, 2. Drift length of the electron, and 3. Energy distribution ofthe emitted electrons.

1. Strong Electric Field Effect at the Surface

In the diode formed by using porous silicon obtained by anodizing then-type single-crystal silicon substrate as described in the conventionalexample (hereinafter, referred to as a porous silicon diode), anelectroluminescence (hereinafter, referred to as EL) light emission isfirst observed in the low voltage range which is insufficient for coldelectron emission. In this light emission mechanism, it is important howthe holes with which electrons recombine generate. Judging from theanalysis of the EL light emission characteristics, the two process, thetunneling of the electron from the valence band of the micro-crystalsilicon layer into the neighboring conduction band thereof and theelectron avalanche due to impact ionizaion, are proposed for thegeneration mechanism of holes (T. Oguro et al, J. AppL Phys. 81 (1997)1407-1412).

These two processes require the strong electric field. Judging from theestimation based on the measurement of the dependency on the excitationwavelength of PL quenching due to the applied electric field, the strongelectric field having an intensity of about 10⁶ V/cm lies in therelatively shallow area from the surface of the porous silicon layer tothe depth of several hundred nm in the porous silicon diode during theEL light emission. Since the electron emission requires an appliedvoltage much higher than that required for the EL, it is supposed thatthe hot electrons are concerned with the electron emission.

On the contrary, in this embodiment, since the oxide layer is formedparticularly intensively on the surface of the RTO-PPS layer by a RTOprocess, like the case of the porous layer, the strong electric fieldgenerated in the vicinity of the surface causes the generation of thehot electrons and the electron emission due to tunneling.

2. Drift Length of the Electron

Based on the measurement results of the time-of-flight of carriers whichis related to a photconductive effect of the porous silicon layer, itwas reported that the drift length of the carriers in the porous siliconlayer was as long as about 1 μm under a strong electric field (10⁵ V/cm)(R. Sedlacik et al, Thin Solid Films 255(1993) 269-271).

This value is much larger than the size of the micro-crystal siliconlayer in the porous silicon layer and it means that the conductiveelectrons can be made easily to be hot electrons. That is, what controlsthe conduction of electrons in the porous silicon layer is not only thesingle-crystal silicon structure itself, but also the surface layer ofthe micro-crystal silicon layer and the interfacial structure such as athin silicon oxide film between the micro-crystal silicon layers where astrong electric field lies.

These fact are applicable to the RTO-PPS layer 6 in this embodiment. Inthe case that an electric field having the similar intensity lies, it issupposed easily that the drift length of the electrons is sufficientlylong as compared with the grain size of the polysilicon 61 (200 nm to300 nm in this embodiment) and the electrons reaching the surface turninto hot electrons.

3. Energy Distribution of Emitted Electrons

Distribution of energy N(E) of electrons emitted from the field emissionelectron source 10 of this embodiment was measured and the result isshown in FIG. 9. In FIG. 9, curve A shows the distribution when DCvoltage Vps is 12V, curve B shows the distribution when the DC voltageVps is 15V and curve c shows the distribution when the DC voltage Vps is18V.

From FIG. 9, it is found that the distribution of energy N(E) ofelectrons is relatively broad and, moreover, includes high-energycomponents of several electron volts, while the peak energy shiftstoward higher energy as the DC voltage Vps applied increases. Thereforeit is supposed that there occurs less scattering of electrons in theRTO-PPS layer 6, and that the electrons which have reached the surfaceof the RTO-PPS layer 6 are hot electrons having sufficient energy. Thatis, it supposed that the quasi-ballistic electron emission phenomenonoccurs.

The fact electrons having reached the surface are not subject to such astrong scattering that causes relaxation to the thermal equilibriummeans less energy loss, i.e. heat generation in the RTO-PPS layer 6 andthe diode current Ips can be maintained constant. Moreover, the columnarpolysilicon 61 remaining in the RTO-PPS layer 6 (see FIG. 1) contributesto the heat diffusion, so that the popping noise is suppressed.

In this embodiment, the polysilicon layer 3 having a columnar structuredeposited on the n-type silicon substrate 1 is anodized, but as long asthe structure shown in FIG. 1 is obtained finally, the bulk polysiliconlayer may be deposited and be anodized. Also, instead of the depositionof the polysilicon layer 3, the n-type substrate may be micro-treatedinto the columnar structure from the principal surface down to apredetermined depth on the surface side and then anodized.

While the n-type silicon substrate 1 ((100) substrate having aresistivity of about 0.1 Ωcm) is used for the electrically conductivesubstrate in this embodiment, the electrically conductive substrate isnot limited to the n-type silicon substrate and, for example, a metalsubstrate such as a chromium substrate, or a glass substrate with aconductive thin film such as an electrically conductive transparent thinfilm of, for example, indium tinoxide (ITO), platinum or chromiumconductive film formed thereon may be used, in which case it is madepossible to achieve larger emission area and lower production cost thanin the case of using a semiconductor substrate such as n-type siliconsubstrate.

Where the conductive substrate is a semiconductor substrate, thepolysilicon layer 3 may be formed on the conductive substrate by the useof LPCVD (Low Pressure Chemical Vapor Deposition) process, sputteringprocess or so on. Also the polysilicon layer may be formed by annealingan amorphous silicon layer formed on a conductive substrate byplasma-CVD process and crystalizing said layer. Where the conductivesubstrate is the combination of the glass substrate and the conductivethin film, the polysilicon layer 3 may be formed on the conductive thinfilm by annealing with an excimer laser to an amorphous silicon layerformed on the conductive thin film by CVD process. It is not limited toCVD process, the polysilicon layer 3 may be formed by CGS (ContinuousGrain Silicon) process, catalytic CVD process, or so on. Where thepolysilicon layer 3 is deposited on the substrate by CVD process or soon, the polysilicon layer to be deposited is influenced extremely by theorientation of the substrate. Therefore, where the polysilicon lyaer 3is deposited on the substrate other than the (100) single-crystalsilicon substrate, such deposition conditions may be set that thepolysilicon grows in the perpendicular direction to the principalsurface

In the above-mentioned embodiment, the PPS layer 4 and the polysiliconlayer 3 are oxidized by a rapid thermal oxidation technique, but theoxidization is not limited to a rapid thermal oxidation, and chemicaloxidation or oxygen plasma oxidation can be used. Instead of oxidation,nitriding can be used. In such a case, nitrogen plasma nitriding,thermal nitriding, or so on can be used. That is, a silicon nitride mayemployed as an insulating film, instead of the insulating film composedof a silicon oxide film 64 as shown in FIG. 1.

Also, in the above-mentioned embodiment, a gold thin film 7 is used as athin metal film. However, the thin metal film is not limited to the goldthin film 7, and may be prepared from any suitable material as far asthe work function of such suitable material is small. Aluminum, chrome,tungsten, nickel, platinum can be used therefor. The work function ofgold is 5.10 eV, that of aluminum is 4.28 eV, that of chrome is 4.50 eV,that of tungsten is 4.55 eV, that of nickel is 5.15 eV, that of platinumis 5.65 eV.

Second Embodiment

FIG. 10 is a schematic diagram showing the configuration of a fieldemission electron source 110 according to this embodiment, FIGS. 11A to11C are cross sectional views of major steps of producing the fieldemission electron source 110. In this embodiment, an n-type siliconsubstrate 101 having a resistivity nearly similar to that of theconductor(for example, (100)-substrate having a resistivity of about 0.1Ωcm) is used for an electrically conductive substrate.

As shown in FIG. 10, the field emission electron source 110 according tothis embodiment includes a strong electric field drift part 106 formedon the principal surface side of the n-type silicon substrate 101 and asurface electrode 107 of a thin metal film which is formed on the strongelectric field drift part 106. And an ohmic electrode 102 is formed onthe back surface of the n-type silicon substrate 101.

For the field emission electron source 110 of this embodiment, thesurface electrode 107 is disposed in the vacuum and a collectorelectrode (not shown) is disposed at a position so as to confront thethin metal film. When a DC voltage is applied with the surface electrode107 at a positive polarity with respect to the ohmic electrode 102 and aDC voltage is applied with the collector electrode at a positivepolarity with respect to the surface electrode 107, electrons injectedfrom the n-type silicon substrate 101 into the strong electric fielddrift part 106 are drifted in the strong electric field drift part 106and emitted through the surface electrode 107. In this case, the currentflowing between the surface electrode 107 and the ohmic electrode 102 iscalled the diode current and the current flowing between the collectorelectrode and the surface electrode 107 is called the electron emissioncurrent. The efficiency of electron emission increase as the ratio ofthe electron emission current to the diode current increases. Theelectrons can be emitted with the field emission electron source 110,even when a DC voltage of as low as about 10 to 20 V is applied betweenthe surface electrode 107 and the ohmic electrode 102.

The strong electric field drift part 106 according to the presentembodiment comprises a drift region 161 of which the cross section atthe right angles to the direction of thickness of the n-type siliconsubstrate 101, an electrically conductive substrate, is in the structureof mesh and in which the electrons are drifted, and a heat radiationregion 162 which is filled in the crystals like openings of themesh-like drift region and which has a heat conductivity higher thanthat of the drift region 161. That is, the heat radiation region 162 isformed in the pillared structure in the parallel direction to thedirection of thickness of the n-type silicon substrate 101. In thiscase, the drift region 161 is made of oxidized porous silicon and theheat radiation region 162 is made of oxidized single-crystal silicon.

Thus, in the field emission electron source 110 of this embodiment,because heat generated in the drift region 161 is radiated through theheat radiation region 162, the popping phenomenon is not observed duringthe electron emission and the stable electron emission can be achievedwith a high efficiency.

A method of making the field emission electron source will be describedbelow with reference to FIG. 11.

First the ohmic electrode 102 is formed on a back surface of the n-typesilicon substrate 101, and then photoresist is applied to the principalsurface of the n-type silicon substrate 101. Said photoresist ispatterned with a photomask A shown in FIG. 13 to form a resist mask 103,resulting in the structure as shown in FIG. 12A. The photomask M isconstructed in such a structure that the plane shape of the resist mask103 is a generally minute square (for example, in the order of 0.1 μm).The photomask M may be constructed in such a structure that the planeshape of the resist mask 103 is a minute polygon, minute circle, minutestar and the like other than a square.

Then the n-type silicon substrate 101 on the principal surface sidethereof is subjected to anodization with a constant current while beingirradiated with light. During this anodization, a liquid electrolytemade by mixing a 55 wt % aqueous solution of hydrogen fluoride andethanol in a proportion of about 1:1 is used and a platinum electrode(not shown) is used as a negative electrode and the n-type siliconsubstrate 101 (ohmic electrode 102) is used as a positive electrode. Bythis anodization, the region which is not covered with resist mask 103on the principal surface side of the n-type silicon substrate 101 ismade porous and a porous layer 111 made of porous silicon is formed,resulting in the structure as shown in FIG. 11B. In FIG. 11B, referencenumeral 112 designates a semiconductor layer composed of a part of then-type silicon substrate 101. The semiconductor layer 112 is in thestructure of a square pole. In this embodiment, the anodization processwas conducted under conditions of a constant current density of 10mA/cm² and duration of anodization being 30 seconds, while irradiatingthe principal surface of the n-type silicon substrate 101 with light bymeans of a 500W tungsten lamp during the process of anodization. Theseconditions are proposed as an example and are not limited thereto. Inthis embodiment, the region on the principal surface side of the n-typesilicon substrate 101 also serves as a semiconductor region.

Then, by effecting the rapid thermal oxidation (RTO) to the porous layer111 and the semiconductor layer 112, a strong electric field drift part106 is formed. Thereafter, the surface electrode 107 made of a gold thinfilm is formed by, for example, deposition on the strong electric fielddrift part 106, resulting in the structure as shown in FIG. 11C. In FIG.11C, reference numeral 161 designates aporous layer 111 oxidized byrapid thermal oxidation corresponding to the above-mentioned driftregion 161 and reference numeral 162 designates a semiconductor layer112 oxidized by rapid thermal oxidation corresponding to theabove-mentioned heat radiation region 162. That is, the strong electricfield drift part 106 is composed of the drift region 161 and the heatradiation region 162 in FIG. 11C. The rapid thermal oxidation processwas conducted at an oxidation temperature of 900° C. for the oxidationperiod of one hour. While the thickness of the surface electrode 107 isabout 10 nm in this embodiment, the thickness is not limited to aparticular value. While the metal thin film (for example, thin goldfilm) serving as the surface electrode 107 is formed by evaporation inthis embodiment, the method of forming the thin metal film is notlimited to evaporation and the thin metal film may be formed bysputtering. The field emission electron source 110 forms a diode withthe surface electrode 107 serving as a positive electrode (anode) andthe ohmic electrode 102 serving as a negative electrode (cathode). Thecurrent which flows when a DC voltage is applied between the positiveelectrode and the negative electrode is diode current.

In the field emission electron source 110 produced by theabove-mentioned process, less change of the electron emission currentwith change in time and no popping noise was observed and the electronswere emitted with a high stability and a high efficiency. In this fieldemission electron source 110, because the dependency of the electronemission characteristic (for example, electron emission current) on thedegree of vacuum is low and the satisfactory electron emissioncharacteristic can be achieved even with a low degree of vacuum, it isnot necessary to use the field emission electron source under a highdegree of vacuum, and it is made possible to produce an apparatus whichuses the field emission electron source 110 at a lower cost withhandling thereof made easier.

In the field emission electron source 110 of this embodiment, becausethe anodization is effected after the resist mask 103 is disposed on thesemiconductor region of the n-type silicon substrate 101, the n-typesilicon substrate 101 is made porous along the direction of thickness inthe area which is exposed. Therefore, judging from the control ofelectric conductivity and the structural and heat stability, the strongelectric field drift part 106 in the field emission electron source 110of this embodiment is supposed to have a better characteristic than thatof the strong electric field drift layer obtained by make the entiresurface of the single-crystal silicon substrate on the principal surfaceside thereof porous, conventionally.

Therefore, in the field emission electron source 110 according to thisembodiment, the electrons seem to be emitted in the following mechanism.When the DC voltage applied to the surface electrode 107 which is of apositive polarity with respect to the n-type silicon substrate 101(ohmic electrode 2) reaches a predetermined threshold value, electronsare injected from the n-type silicon substrate 101 into the strongelectric drift part 106 by thermal excitation. On the other hand, thereare a large number of micro-crystal silicon layers havingnano-structures where the quantum confinement effect occurs in the driftregion 161 of the strong electric field drift part 106, and on thesurface of the micro-crystal silicon layer, the silicon oxide filmhaving a thickness smaller than that the crystal grain size of themicro-crystal silicon layer is formed. At this time, since most ofelectric field applied to the strong electric field drift part 106 isapplied across the silicon oxide layer formed on the surface of themicro-crystal silicon layer, the injected electrons are accelerated bythe strong electric field applied across the silicon oxide film and aredrifted in the drift region 161 toward the surface. In this case, thedrift length of the electrons is very long as compared with the grainsize of the micro-crystal silicon layer, the electrons reach the surfaceof the drift region 161 with almost no collision. The electrons whichhave reached the surface of the drift region 161 are hot electronshaving a much higher kinetic energy several kT or much more higher thanthat in the state of thermal equilibrium and easily penetrate thesurface electrode 107 through the oxide layer at the top surface of thedrift part 106 due to tunneling, thereby to be emitted to into thevacuum.

In the field emission electron source 110 of this embodiment, theelectrons can be emitted without the occurrence of the popping noise andwith a high efficiency and a high stability. This is because it issupposed that heat generated in the drift region 161 of the strongelectric field drift part 106 by applying voltage transmits along theheat radiation region 162 and radiates to the outside, therebytemperature rise being suppressed.

Based on the discussion above, it is supposed that the strong electricfiled drift part 106 has a semi-insulating characteristic to make astrong electric field lie. And it is supposed that in the strongelectric filed drift layer 106, the electron scattering is small and thedrift length is long, the heat conductivity being high enough tosuppress the thermorunaway of the diode current. Therefore, it is madepossible to achieve stable electron emission with high efficiency.

The facts that support the mechanism of the electron emission due to thetunneling effect of the hot electrons are as described above. Such factsare 1. Strong electric field effect at the surface and 2. Drift lengthof the electron.

Thus, based on the above-mentioned facts 1 and 2, it is supposed thatthe electrons are emitted due to tunneling of the hot electrons in thefield emission electron source 110 of this embodiment.

In this embodiment, as described above, a part of the n-type siliconsubstrate 101 on the principal surface side thereof serves as asemiconductor region and this semiconductor region is anodized. However,any one of single-crystal silicon, poly-crystal silicon, amorphoussilicon, single-crystal silicon carbide (SiC), poly-crystal siliconcarbide or amorphous silicon carbide or so on may be laminated as asemiconductor region on the n-type silicon substrate and thenanodization may be effected. Also, the electrically conductive substrateis not limited to the n-type silicon substrate and, for example, a metalsubstrate such as a chromium substrate, or a glass substrate with aconductive thin film such as an electrically conductive transparent thinfilm of, for example, indium tin oxide (ITO), platinum or chromiumconductive film formed thereon may be used, in which case it is madepossible to achieve larger emission area and lower production cost thanin the case of using a semiconductor substrate such as n-type siliconsubstrate. Where the conductive substrate is a semiconductor substrate,the polysilicon layer may be formed on the conductive substrate by theuse of LPCVD process, sputtering process or so on. Also the polysiliconlayer may be formed by annealing an amorphous silicon layer formed on aconductive substrate by plasma-CVD process and crystalizing said layer.Where the conductive substrate is the combination of the glass substrateand the conductive thin film, the polysilicon layer may be formed on theconductive thin film by annealing with an excimer laser to an amorphoussilicon layer formed on the conductive thin film by CVD process. It isnot limited to CVD process, the polysilicon layer may be formed by CGS(Continuous Grain Silicon) process, catalytic CVD process, or so on.Where the polysilicon layer is deposited on the substrate by CVD processor so on, the polysilicon layer to be deposited is influenced extremelyby the orientation of the substrate. Therefore, where the polysiliconlayer is deposited on the substrate other than the (100) single-crystalsilicon substrate, such deposition conditions may be set that thepolysilicon grows in the perpendicular direction to the principalsurface of the substrate.

In the above-mentioned embodiment, a gold thin film is used as a thinmetal film to form a surface electrode. However, the thin metal film isnot limited to the gold thin film, and may be prepared from any suitablematerial as far as the work function of such suitable material is small.Aluminum, chrome, tungsten, nickel, platinum can be used therefor.

When a part of the semiconductor region is made porous by anodization,such a magnetic field is applied to the n-type silicon substrate 101that the rate of making the semiconductor region porous in theperpendicular direction to the principle surface of the n-type siliconsubstrate 101 which is an electrically conductive substrate is muchfaster than that in the other direction, with the result that theanisotropy in the rate of making porous is enhanced. Thus, because theanisotropy in the rate of forming the porous layer during anodization ofthe region which is to be a drift region 161 by above-mentioned rapidthermal oxidation is enhanced, the shape of the drift region 161 in thehorizontal direction and in the direction of thickness can be controlledin a better manner and the minute pattern of heat radiation region 162and the drift region 161 can be formed in the direction of thicknesswith a good controllability. In this case, to enhance the anisotropy,the magnetic field may be applied across the n-type silicon substrate101 in the vertical direction.

Third Embodiment

The field emission electron source 110 of this embodiment has aconfiguration as shown in FIG. 13 and the basic configuration thereof issubstantially similar to that of the second embodiment. Therefore, onlythe difference from the second embodiment will be described below forthe sake of brevity.

The field emission electron source 110 of this embodiment ischaracterized by the structure of the drift region 161 in the strongelectric field drift part 106 shown in FIG. 13. In this embodiment, thedrift region 161 is in the laminated structure (multi-layered structure)made by alternately laminating a first drift layer 161 b having a higherporosity and a second drift layer 161 a having a lower porosity in sucha manner that a second drift layer 161 having a lower porosity is formedon the front surface of the drift region 161. Components similar tothose of the second embodiment will be denoted with the same referencenumerals. The description thereof will be omitted.

Thus, since the field emission electron source 110 of this embodimentcomprises a drift region 161 in the multi-layered structure as describedabove, the overflow of the diode current can be suppressed moreeffectively and the efficiency of the electron emission can be enhanced,compared with the field emission electron source of the firstembodiment.

The process of producing the field emission electron source of thisembodiment is almost the same as that in the second embodiment and onlythe conditions of anodization are different. That is, in thisembodiment, anodization under the first condition with current densitybeing small and anodization under the second condition with currentdensity being large are repeated alternately. At the time whenanodization under the frist condition is completed one time, a porouslayer having a low porosity is formed on the surface of the n-typesilicon substrate. Then at the time when anodization under the secondcondition is completed, a porous layer having a high porosity is formedon one side of said porous layer having a low porosity adjacent then-type silicon substrate 101.

Fourth Embodiment

The field emission electron source 110 of this embodiment has aconfiguration as shown in FIG. 14 and the basic configuration thereof issubstantially similar to that of the second embodiment. Therefore, onlythe difference from the second embodiment will be described below forthe sake of brevity.

The field emission electron source 110 of this embodiment ischaracterized by the structure of the drift region 161 in the strongelectric field drift part 106 shown in FIG. 14. In this embodiment, thedrift region 161 is a layer whose porosity changes continuously in thedirection of thickness. In this case, the porosity increasescontinuously from the front surface toward the n-type silicon substrate101. Components similar to those of the second embodiment will bedenoted with the same reference numerals and the description thereofwill be omitted.

Thus, since the field emission electron source 110 of this embodimentcomprises a drift region 161 whose porosity changes continuously asdescribed above, the overflow of the diode current can be suppressedmore effectively and the efficiency of the electron emission can beenhanced, compared with the field emission electron source of the secondembodiment.

The process of producing the field emission electron source of thisembodiment is almost the same as that in the second embodiment and onlythe conditions of anodization are different. That is, in thisembodiment, the current (current density) is changed continuously duringanodization, with the result that the porosity of the porous layerdescribed in the second embodiment changes continuously.

For example, the current density is increased (gradually) as time passesfrom the time when the anodization is started. At the time when theanodization is completed, a porous layer having the porosity increasingcontinuously in the direction of thickness from the front surface towardthe n-type silicon substrate 101 is formed. The resulting porous layeris oxidized by a rapid thermal oxidation process to form a drift region161 whose porosity changes continuously.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof and the accompanying drawings, itis to be noted that various changes and modifications are apparent tothose skilled in the art. Such changes and modifications are to beunderstood as included within the scope of the present invention asdefined by the appended claims unless they depart therefrom.

What is claimed is:
 1. A field emission electron source comprising: anelectrically conductive substrate having principal surfaces; a strongelectric field drift layer formed on one of the principal surfaces ofsaid electrically conductive substrate, comprising at least a)semiconductor crystal regions formed in a manner to stand up verticallyon said one of the principal surfaces of said electrically conductivesubstrate, and b) interspersed between said semiconductor crystalregions, semiconductor micro-crystal regions having nano-structures witha first insulating film having a thickness smaller than that of amicro-crystal of said semiconductor micro-crystal regions formed on asurface of said micro-crystal; and a surface electrode of a thinconductive film formed on said strong electric field drift layer,wherein when a voltage is applied to make said surface electrode apositive electrode with respect to said electrically conductivesubstrate, electrons injected from said electrically conductivesubstrate are drifted in said strong electric field drift layer and areemitted through said surface electrode.
 2. The field emission electronsource of claim 1, wherein said strong electric field drift layercomprises at least a) drift regions for drifting electrons therethroughand b) heat radiation regions having a heat conductivity better thanthat of said drift regions, both regions being mixed and distributeduniformly on said one of the principal surfaces of said electricallyconductive substrate.
 3. The field emission electron source of claim 1,wherein said strong electric field drift layer is a layer made byalternately laminating layers whose porosities are different from eachother in a direction of thickness of the electrically conductivesubstrate or is a layer whose porosity changes continuously in saiddirection of thickness.
 4. The field emission electron source of claim2, wherein said drift regions and said heat radiation regions are madeof any one of a silicon or silicon carbide single-crystal, a silicon orsilicon carbide poly-crystal, and an amorphous silicon or siliconcarbide.
 5. The field emission electron source of claim 1, wherein saidsemiconductor micro-crystal regions are made of a porous semiconductormaterial obtained by anodization.
 6. The field emission electron sourceof claim 2, wherein said heat radiation regions are covered on thesurface thereof by a second insulating film selected from an oxide filmand a nitride film.
 7. The field emission electron source of claim 1,wherein said first insulating film is selected from an oxide film and anitride film.
 8. The field emission electron source of claim 1, whereinthe electrically conductive substrate is a substrate on a principalsurface of which an electrically conductive film is formed.
 9. The fieldemission electron source of claim 1, wherein said surface electrodecomprises a thin metal film.
 10. The field emission electron source ofclaim 1, wherein said surface electrode comprises a thin gold film. 11.The field emission electron source of claim 1, wherein said electricallyconductive substrate has an ohmic electrode on a back surface thereof.12. The field emission electron source of claim 1, wherein saidelectrically conductive substrate comprises an n-type silicon substrateand said strong electric field drift layer is formed from an undopedpolysilicon layer.
 13. The field emission electron source of claim 1,wherein no popping phenomenon occurs during electron emission.
 14. Avacuum chamber, said vacuum chamber housing the field emission electronsource of claim 1 and a collector electrode.